Recently, an electric motors often adopt a type of drive by an inverter of a pulse width modulation type (hereinafter, referred to as PWM type). In the case of the inverter driving of the PWM type, since the neutral point potential of a coil does not become zero, the potential difference (hereinafter, referred to as the axial voltage) is generated between the outer ring and the inner ring of a bearing. The axial voltage includes a high frequency component by the switching. Thus, when the axial voltage reaches the insulating breakdown voltage of an oil film in the bearing, the minute current flows in the bearing, whereby the electrolytic corrosion is generated in the bearing. In a case where the electrolytic corrosion proceeds, a wave-shaped abrasion phenomenon is generated in the bearing inner ring or the bearing outer ring or the bearing ball and may lead to an abnormal sound, which becomes one main cause of defects in the electric motor.
In the related art, in order to prevent the electrolytic corrosion, the following measures are considered.
(1) to put the bearing inner ring and the bearing outer ring in a conduction state.
(2) to put the bearing inner ring and the bearing outer ring in a insulating state.
(3) to reduce the axial voltage.
As a specific method of (1), a lubricant for the bearing can be conductive. The conductive lubricant has a problem in that the conductivity deteriorates as time goes on or the sliding reliability is insufficient. Moreover, a method of installing a brush on the rotation axis, making the conduction state can also be considered, but this method also has a problem such as a brush abrasion powder or a need for a space.
As a specific method of (2), changing a steel ball in the bearing to a ceramic ball can be included. This method has a very high electrolytic corrosion prevention effect, but has a problem of high costs; therefore it cannot be adopted in a general-purpose electric motor.
As a specific method of (3), a method of reducing the axial voltage by short-circuiting a stator steel core and a bracket to change the electrostatic capacity is well known in the related art (for example, see Patent Citation 1).
Impedance when the electrostatic capacity and the resistance are connected with each other in parallel is indicated by the relation expression of Z=1/jωC+R. Herein, Z refers to impedance, j refers to an imaginary number, ω is a angular frequency, C refers an electrostatic capacity, and R refers to a resistance. As is apparent from the expression, if the electrostatic capacity rises or the resistance drops, the impedance drops. On the contrary, if the electrostatic capacity drops and the resistance rises, the impedance rises.
In Patent Citation 1, the impedance of the stator side is reduced by short circuiting the stator steel core and the bracket, whereby the electrolytic corrosion of the bearing is prevented.
That is, generally, in an electric motor, which is used in places where water is used such as washing machines and a dish washing and drying machine and which has a fear of an electric shock, it is necessary to add an separate insulation (hereinafter, referred to an additional insulation) in addition to the insulation of a charge portion (hereinafter, referred to as basic insulation). On the other hand, except for those situations, since electric motors used in indoor air conditioners, outdoor air conditioners, water heaters, air cleaners or the like do not have a fear of electric shock, the additional insulation is unnecessary. Thus, since the electric motors used in the indoor air conditioners, outdoor air conditioners, water heaters, air cleaners or the like do not have the rotor of the insulation configuration, the impedance of the rotor side (the bearing inner ring side) is in a low state. On the contrary, since the stator side (the bearing outer ring side) has the insulation configuration, the impedance is in a high state. In this case, since the potential of the bearing inner ring side is high, while the potential of the bearing outer ring side is low, there is an unbalance state, whereby the high axial voltage is generated. In addition, there was a possibility in which the electrolytic corrosion could be generated in the bearing due to the high axial voltage.
In order to avoid this state, Patent Citation 1 adopts a method of lowering the impedance of the stator side (the bearing outer ring side) as described above by short circuiting the stator steel core and the bracket, thereby approximating the impedance of the rotor side (the bearing inner ring side).
However, the method of the related art like Patent Citation 1 had the following problems. That is, since the method of the related art is a method of short-circuit, adjustment of the impedance is impossible, and the axial voltage rises depending on the material or the structure of the magnet of the rotor. Furthermore, as another problem, as a method of lowering the impedance, a state in which balance is maintained between the bearing inner ring and the bearing outer ring with the high potential can be included. In the case of this condition, a case where, if the balance of the impedance collapses due to the usage circumstances of the electric motor, an imbalance of the assembly accuracy of the stator and the rotor, or the like, and conversely, the axial voltage rises and the electrolytic corrosion is easily generated, was also considered as the possibility.
As described above, according to the electric motor of the present invention, the impedance of the rotor side (the bearing inner ring side) is raised to approximate the impedance of the stator side (the bearing outer ring side), whereby it is possible to hold the balance of the high frequency potential of the bearing inner ring side and the bearing outer ring side. Thus it is possible to provide an electric motor which prevents the occurrence of electrolytic corrosion in the bearing and electric machinery equipped therewith.    [Patent Citation 1]: Japanese Patent Unexamined Publication No. 2007-159302